Field of the Invention
The present invention relates to X-ray devices, and in particular to a stereo X-ray subtraction apparatus.
A stereo X-ray installation is known from European Patent application No. A-0088356 which has a stereo X-ray tube having foci which are alternately activated. Two X-ray beams are thus generated from the two foci disposed at a small distance from each other, the central rays of the X-ray beams intersecting in the radiography plane. The resulting image is supplied to an X-ray image intensifier, having an inlet fluorescent screen on which the radiation images, generated in chronological succession by both foci, appear. The output of the X-ray image intensifier is intensified to a television camera for displaying the resulting image. A central control unit for controlling the television chain, including a processing circuit with image storage means for storage of X-ray stereo television images is supplied. The processing circuit also includes subtraction stages for undertaking subtraction from a stored video signal and signal chronologically following the stored video signal. Stereo X-ray devices of this type are employed in order to spatially view X-ray images for the purpose of, for example, precisely monitoring the insertion and positioning of a catheter in a subject.
The stereo installation disclosed in the aforementioned European patent application irradiates the subject from various directions. The radiation images striking the inlet fluorescent screen of the X-ray image intensifier are detected by a television camera and are alternatingly read into two image memories in the processing circuitry, such that radiographs without a contrast agent are stored in a first memory pair, and subsequently subtraction radiographs are stored in a second memory pair. The memory contents are supplied to a stereo display device for viewing by an Examiner.
A problem in the use of such stereo technology is that in a video signal of a stereo partial image, residual information from the preceding other stereo partial image exists. This may be present in an amount up to 30%, so that the residual information is a substantial source of interference for the following subtraction operation.
A noise reduction system for a television installation is disclosed in U.S. Pat. No. 4,064,530 wherein an image is averaged over several scannings by means of recursive filtering, the averaged image being read into a memory. For this purpose, a current video signal is multiplied by a factor (1-a) and the stored video signal is multiplied by a factor a. The current video signal is thus attenuated and superimposed with the also attenuated stored video signals so that the noise, for example, introduced by a television system is reduced, as well as the quantum noise.